An LEO may arrest someone, or detain the person for investigation, if the LEO has probable cause to believe the person has committed a crime.
 "Probable cause" is "sufficient reason based upon known facts to believe a crime has been committed or that certain property is connected with a crime...."
 An NFA weapon may only be lawfully possessed by an LEO, licensed dealer/manufacturer or an individual who has satisfied the requirements of the NFA for private ownership and who has properly registered the firearm as required under the NFA. Except for such persons, possession of an NFA weapon is a crime.
 If a person has in openly in his possession a weapon that is obviously an NFA weapon, he is committing a crime, unless he satisfies the requirements for lawful possession of the NFA weapon.
 The fact of the person's possession of the weapon are open and obvious. Any fact that might suggest that such person's possession is lawful, are not open and obvious.
 The facts known to an LEO observing such person, i. e., the possession of an NFA weapon, can reasonably cause the LEO to believe that a crime is being committed, i. e., the unlawful possession of an NFA weapon. Any facts which could possibly lead the LEO to conclude that the possession of the NFA weapon is actually lawful are hidden and therefore not known to the LEO.
 Therefore, based on the facts known to the observing LEO at the time. he has been led to believe that the person in possession of the NFA weapon is committing a crime, i. e., unlawful possession of an NFA weapon.
 If the LEO detains the individual possessing the NFA weapon, and that person cannot provide evidence of the lawful possession of the NFA weapon, based on the known facts, the LEO continues to have reason to believe the person is illegal possession of an NFA weapon. That constitutes probable cause for arrest.
 If the person can supply evidence that he is in lawful possession of the NFA weapon, the known facts no longer support the belief that the person has committed a crime. Now the LEO would no longer have probable cause for arrest.
Well put sir as all your previous postings have been...
I only have one problem with number 1 and that is a cop (usually a SOB PIG) can do whatever they feel like doing to you whether they have justifiable or probable cause if they feel like it, they had a bad day, they're hungover, they're the only law in the land i.e. small town sheriffs or marshals and it's now up to you to prove your innocence not them proving your guilt...Presumption of Innocence is a thing of the past…Think pre Miranda, TN/MS/AR local yokel/county mountie…
Guys, if you're at the range and shooting a sub-machine gun (legal of course) and a fully uniformed LEO is at the slot three over and decides that he wants to hassle you--because your ejected cases have been bouncing into his space and asks you for proof of legal ownership/possession what do you say? "Sorry, I only have to show that to a BATFE agent" You could well end up cow hopping back to the front door, handcuffed and taken to the station house and possibly booked with possession of a illegal firearms, machine gun or whatever damn term your state/county/city uses...Are you innocent, yes, but you’ll still probably have to retain counsel to get your (legal) gun back…Read Numbers 4 through 7 above…
Same range and the Range Master, manager/owner comes over as says (for whatever, valid to him, reasons) “unless you have proof of legal ownership of said weapon, I’ll have to ask you to leave”, how hard is it to have a copy of the papers with you to continue shooting…Is he just supposed to take your word for it that it’s properly licensed and registered—maybe not you but other people have been known to lie!
You have to have proof of registration and insurance when you drive a vehicle, oh, and a valid license as well otherwise the officer can and will tow your car away, whether you’re the legal owner or not…You don’t even have to be pulled over for cause but just driving through one of these, now common, roadside alcohol testing stations…I got pulled over by a State Highway Patrol, lights flashing, for driving a stolen car, guns pointed at me, screamed at to get out of the car, handcuffed, all because someone transposed a couple of letters in my license plate—make matters worse, not only was I from out of state but out of country…Showed them my proofs of ownership, insurance and license and I was on my way but I still had guns pointed at me just because we used the same number/letter system of registering vehicles…